Inertial navigation systems are based upon the stabilization of a coordinate frame defined by gyroscopes and the measurement and processing of accelerometer information in that frame to achieve velocity and position knowledge of the vehicle. The strapped down mode of achieving the reference measurement frame described herein uses gyroscopes "strapped" to the vehicle but containing one degree of freedom. The rate measurements are appropriately integrated and compensated and computationally added to the angle output to derive the stabilized attitude of the reference frame. To permit highly accurate attitude measurements with vehicle rates of 0.01.degree./Hr thru 500.degree./sec a dynamic performance over the range 2.times.10.sup.8 :1 is required. One purpose of this invention is to reduce the dynamic range requirement significantly and secondly to make use of this approach to permit a unique method of calibrating the gyro scale factor, symmetry, and other terms which are critical parameters in strapdown system performance. The requirements upon the gyroscope to have accurate measurement capability from one day to the next for months or even years while still capable of operating over the dynamic range is severe. It would be advantageous to be able to move the gyro in an accurate manner relative to a known rate (usually earth's rate) so as to calibrate the gyro. This calibration could be accomplished via the degree of freedom, periodically relieving the gyro of the requirement to maintain stability over long periods of time. Additionally with the gyroscope appropriately skewed relative to a degree of freedom, the gyro scale factors can be easily calibrated either in the factory or in the field by causing a rotation around this free axis, enunciating the output via an angular transducer and comparing this to the integrated gyro output.
As stated previously, a significant problem that strapped down systems impose upon the inertial sensors is the requirements for long term stability as well as accurate scale factors over a wide measurement range. Generally, gyroscopes and accelerometers tend to have excellent short term stabilities measured over a period of time the unit is operating, usually up to 10 hours. This is better by a factor of 5 or 10 to 1 than long term stability. Long term stability is measured over periods of weeks and months with many turn-ons and turn-offs between. Gyroscopes and accelerometers used in strapdown systems very often cannot meet accuracies commensurate with a 1 NMPH navigation system. In order to reduce the requirement upon the component, calibration procedures are utilized to periodically calibrate the inertial parameters by rotating each component against the earth rate rotation or "g" vector. With strapped down system, the rotation is impossible unless a degree of freedom is allowed.
Making use of the rotational freedom suggested herein in the roll direction, permits the parameters such as gyro biases, scale factor and symmetries, and acceleration biases to be calibratable. The gyro and acceleration axes are mounted so that none will directly lie along the roll axis of freedom. Thus, when the rotational freedom permits a rotation all axes will sense a component of the rotation permitting gyro scale factor calibration. Also this same rotation can be referenced against the vertical component of the earth's rate and the "g" vector to obtain bias updates for both the gyro and accelerometers.
It must be remembered that scale factor is a very dominant term requiring extreme accuracy (50 ppm and better) and the ability to have a simple rotation measured by an angular transducer is extremely beneficial. The total angle is accurately measured by the transducer and compared to the integrated output of the gyro. The comparison yields the information for calibration.
The essence of the invention is to combine the ability to calibrate scale factor and other inertial parameters (by permitting a degree of freedom and sensor skewing relative to the axis of freedom) and to permit this same degree of freedom to exist along the most dynamic axis of an aircraft, usually roll, so the full dynamic range requirement on the sensors do not exist.
A specific characteristic of most aircraft whose aerodynamic performance permits vehicle rates in the order of 200 to 500 degrees per second is that the high rate occurs along one axis only; the roll axis. Further, this rate is transient in nature. It occurs quickly with high angular acceleration and lasts for approximately a second or less. The invention makes use of this characteristic, and suggests the mechanical configuration which permits roll mechanical freedom. With roll mechanical freedom two significant benefits arise. The first is the reduction in dynamic range by at least a half order of magnitude, and the second, the mechanical freedom which permits calibration sequences since rotation is now permissible against a known rate and "g" vector.